Microsoft Announces Achievement Statistics
Done for an article on Gamasutra, Greenberg announced that there have been 2.5 billion achievements unlocked by Xbox Live users. To give you an idea, Kotaku breaks it down even further, adding up the combined achievements to a total of 52 billion gamerscore points. Breaking it down even further, that is approximately 150 achievements per user.
I believe a special mention was made of Minioger and Lowlifespider as well